DENITRIFICATION IN, SECONDARY CLARIFIERS

Denitrification in secondary clarifiers is primarily due to hydrolysis of particulate degradable COD and decay of biomass. The reduction fac tor of these processes under anoxic as compared to aerobic conditions was investigated in the treatment plants of the City of Zurich. The Zu rich-Glatt plant has circular clarifiers with central inlet. The treat ment plant Zurich-Werdholzli has rectangular clarifiers with cross flo w and scrapers with vacuum removal of the sludge. At Zurich Glatt deni trification capacity in the clarifiers was 30% of total denitrificatio n, whereas at Zurich-Werdholzli only 15 % was observed. The sludge mas s in the secondary clarifier depends on inlet flow, return sludge flow , activated sludge concentration, sedimentation properties of the slud ge and scraper intervals. During dry weather and due to good sedimenta tion properties the activated sludge blanket in the thickening zone of the rectangular clarifiers at Zurich-Werdholzli is almost completely removed after passage of the scraper. Denitrification volume in the se condary clarifier can therefore be modelled in relation to the ratio o f inlet to return sludge flow and scraper interval. With the aid of co ntrol of return sludge flow and scraper intervals denitrification capa city in the secondary clarifiers may be improved. In order to control denitrification in the secondary clarifier the model must include a va riable sludge volume in the clarifier related to scraper interval unde r the assumption of constant sludge mass in the entire system.